Posted
by
timothy
on Tuesday November 11, 2008 @09:30AM
from the goosebumps-might-help-evade-them dept.

MediaSight writes "Shortfin mako sharks can shoot through the ocean at up to 50 miles per hour (80 kilometres an hour). Now a trick that helps them to reach such speeds has been discovered — the sharks can raise their scales to create tiny wells across the surface of their skin, reducing drag like the dimples on a golf ball."

Not the same thing. Sharkskin was indeed known for a while to present an extremely slippery surface. What these researchers found is an additional effect: that the sharkskin can raise the individual "teeth" so that a turbulent layer gets created. This turbulent water layer prevents flow dissociation, which in turn reduces drag.

There was a 2-man boat at the last Olympics that was using dimples in its coating rather than the fairly standard sharkskin approach - it didn't win, but it was noted due to its novel

Not the same thing. Sharkskin was indeed known for a while to present an extremely slippery surface. What these researchers found is an additional effect: that the sharkskin can raise the individual "teeth" so that a turbulent layer gets created. This turbulent water layer prevents flow dissociation, which in turn reduces drag.I think it's a bit of both, i.e. previously known, and new info. IIRC, back when the USA won the America's Cup back down in Australian water, people knew that the rough surface on s

I think only the structure of the scales was actually known, sharks raising them to obtain a different skin texture is the new part here.I bet all those athletes who paid handsomely for their "shark scale" suits are regretting that purchase just about, now.

The scale structure has been known for around 20 years - I first saw it in a product at 3M in around 1989 or so, and the guys responsible (it was a plastic film intended to reduce drag) were full of enthusiasm about it.

Being able to change the texture is pretty cool - to replicate that capability would be even cooler.

it's probably from steroids, the same reason athletic records of all kinds have been broken in recent years.

It's not a new phenomenon. Athletic records have been consistently being bettered for as long as they have been keeping them. I'm not denying enhancements, I'm just saying that the facts that records are falling is not by itself proof.

That's not from any fancy suits, it's probably from steroids, the same reason athletic records of all kinds have been broken in recent years.

It's pretty much pointless to compare modern-day athletics to any historical records because of the extreme use of steroids now.

Yes, it's from fancy suits. They didn't just invent steroids for swimmers. They did just invent a new type of suit, and since it's invention records have been falling far faster than before. Before, some potentially-juiced-up swimmer would

Not at all. Supercavitation puts a small high frequency oscillator at the front of the submersible. While cavitation creates small air bubbles which form and collapse, supercavitation produces an entire cavity of air in the water which the submersible now flies through with reduced drag. It's reduction of drag through reduction of medium density.

This method energizes the flow, and induces a premature shift from laminar to turbulent flow. When a laminar flow encounters an adverse pressure gradient (large c

Because their Reynolds number is very big and their boundary layers are already turbulent.
The story is so oversimplified that raising questions from it is just pointless.
The facts are as follow:
1. Roughness tends to increase drag because makes boundary layers turbulent.
2. Turbulent boundary layers do stand higher adverse pressure gradients prior to separation
3. Separation increases drag much more than turbulent boundary layers.
Then, there are some applications where you would have a separated flow, and promoting turbulence through roughness would reduce the drag. This is not the case of aviation. It is not the case for sure of sharks when they are not moving their tails. It may be the case of sharks when they are moving their tails to obtain propulsion.

Actually Boeing was researching doing just that, both as a material change and using compressed air to create a virtual shape which could be controlled to allow a change in resistance for takeoff, landing, and cruise.

Actually, the booms on KC-135R do have these dimples and a raised chaotic surface. I used to repair them when boomers would raise them too quickly and smash them against the retaining hook. It was a bear fixing these surfaces due to the way the boom was shaped, adding in the weird surfaces? A repair that should take an hour took a whole afternoon.

well, not necessary, since submarines Reynolds number (rho*V*L/mu) tends to be much bigger, and it may happen that the boundary layer is already turbulent.
I'm confused with the story itself, it is poorly written.
From my knowledge on fluid mechanics, turbulent boundary layers increase drag unless separation is avoided (turbulent boundary layers stand higher adverse pressure gradients prior to separation). I guess that what happens with sharks is that they can twist their tails with higher energy providing

Back in the 80s we switched from polishing the bottom of our race boat to a glass like finish to spraying it with a gel mixture (as in gel coat, not jello) full of small oblong granules. We found that by spraying it a certain way we could get the particles to more or less line up in the orientation we needed. Careful polishing after the fact gave us the finish we were looking for without destroying this new, textured surface. We did this directly in response to an article I had read about how a sharks skin allows it to move quickly through the water. The article went further to say that this also applied to most all scaled fish.

This modification allowed the boat to break the surface tension of the water more easily when launching from a standing start and added several miles an hour to our top end speed. In a game where every mile an hour might cost 1000s or 10s of thousands of dollars this was *the* most effective modification we had ever done to the boat and one that to this day we joke about because it took our competition many years to figure out.

Step Seven- New Structure on the base
Much like a car tire, your ski has a tread pattern we call the structure. What type of structure works best depends on what kind of snow you typically ski in. Utah shops will run a different pattern on their skis than we do here in California. In any case this is the part that puts the pattern into the b

In all honesty, without knowing the flow conditions existing on the hull surface, there is just as good a chance that you increased drag (increased skin friction with a fully attached flow) as decreased it (tripped the laminar boundary layer into turbulence and delayed flow separation). Without a carefully controlled experiment, you probably just attributed speed increases from outside sources to your new, expensive hull finish.

It's just like my father has always said to me. Every 20-30 years any particular bit of knowledge is "discovered" and it's almost never the first time. Many examples can be found of knowledge being repeatedly discovered and forgotten periodically sometimes dozens of times over decades or centuries. The only reason we've gotten this far now is because we've learned to record and reference information accurately.

I would also venture to point out that scientists did not just "discover" why the mako shark can s

From the article I conclude that the researchers have performed an experiment that indicates that if sharks do raise their scales while swimming it might allow them to go faster. They've discovered nothing about what sharks actually do.

On the back half of a wing, the boundary layer needs sufficient kinetic energy to remain attached to the surface. By sucking off the static air, moving air would take its place and keep the flow laminar out to the trailing edge.
The MEMS devices were small diaphragms inserted into cavities in the wing surface. Rather than remove the static boundary layer, they would oscillate and energize the existing boundary layer, achieving the same effect with considerably less power and substructure.

First, the turbulent layer formed by the raised scales does not act as a buffer and will actually cause more surface drag on the shark than a smoother layer (if the scales were flat, for example).

Second, the scales do not prevent a turbulent wake, they create it.

The way this reduces drag is reasonably straightforward and has to do with the boundary layer.

In an idealized model (no friction) you would calculate that any object has zero drag, or net force from the air acting on it. You would integrate the force of the air pressure acting on all sides of the object and get zero. If you are looking at a circular cross section object, you have high pressure at the leading point, very low pressures at the top and bottom, and high pressure again at the trailing point, for a net of zero drag.

However, what happens (aside from the usually small effect of friction) is that the boundary layer "seperates" from the object, so (back to our circle) you have high pressure in front, low pressure on the sides, and then the boundary layer seperates from the object and you wind up with low pressure in the back, too. So, high pressure in front, low pressure everywhere else, you have drag.

The way that golf balls (and sharks, apparently) attack this problen is to screw with the boundary layer flow. They "trip" the flow (using dimples or raised scales) into a turbulent boundary layer. This boundary layer creates more friction drag than a viscous (smooth) boundary layer, but because the particles in the boundary layer are moving every which way (it's a higher energy boundary layer) it will remain attached to more varying geometries than a viscous boundary layer will, so it won't seperate (or at least it won't seperate as early) from a shape like a golf ball or a shark, so you've reduced pressure drag by increasing viscous drag.

This usually works out in your favor, viscous drag is usually nearly negligible next to pressure drag.

It's also important to mention that it's a speed dependent phenomenon. I remember a discussion about this when they started with dimpled motorcycle helmets that, in air, the effect only works around 100 mph and up, and was useless for the average motorcycle driver. In water, with it's higher density, this regime seems to be effective at much lower speeds, so I haven't seen any dimpled submarines, yet.

As I said, higher density means lower required velocity to transition from laminar to turbulent flow. The change in density is about a factor of 20 higher than the corresponding change in viscosity.
And Reynolds numbers aren't really applicable in transition regimes, but I'm not digging out my BSL.

In water, with it's higher density, this regime seems to be effective at much lower speeds, so I haven't seen any dimpled submarines, yet.

Since most submarines in the world are the military type, would not a dimpled/scaled hull, which would cause turbulence even while allowing the sub to go faster, defeat the whole purpose of having a sub--to go places undetected?

I'm afraid the answer to that question is one of those that gets black helicopters flying over your house;)
My guess is that the hydrodynamic flow around the stern of advanced submarines is so optimized that you don't get a loss of laminar flow along the surface - in that case the dimples would actually make things worse.

The team created artificial shark skin with a 16 x 24 array of synthetic scales, each 2 centimetres in length and angled at 90Â to the surface of the "skin".

This is at least a full order of magnitude larger than the scales on a shark's skin.

According to this source [google.com.au], the kolmogorov scale [wikipedia.org] in the ocean is in the order of 1mm. Therefore, is the effect described in TFA going to actually be present for shark's skin? It seems to me that the effect will be minimal, if it is present at all..

Don't you remember all the cars back in the 80's that had textured vinyl (Landau) tops?

That was a direct response to the fuel crisis of the mid 70's. The pebbled texture of the vinyl roof allowed the boundary layer to remain attached longer and directly reduced the amount of drag on the vehicle, increasing fuel economy my a couple of MPG.

A couple of clued-in NASCAR teams adopted it for their race cars.

Sadly, the vinyl roof was subject to the whims of fashion and styling and died out in the 90's - just in ti

Never thought a vinyl roof had anything to do with fuel economy. Not sure I believe it either. Surely a much easier way to achieve a textured top is texture the metal, or possibly the paint?

Every time we got stuck with a car with a vinyl cover over a metal roof, we'd rip the thing off and paint the metal underneath. Exterior vinyl roofs on cars that already have metal roofs are stupid. I always thought of them as particularly obnoxious fashion whims. High maintenance, adds weight, traps water and rus

You're hilarious. There's no need to do that with vinyl. And the bullshit vinyl roof was just one more thing that had to be thrown away. And that assumes I believe you, anyway. If they really wanted to reduce drag, they would have redesigned the cars of the day not to look like a concrete bunker.

My parent post is actually an obscure reference to a running gag perpetrated during the early 90's by Circle Track magazine - back when it was a technical mag and not "NASCAR People"

One of their readers wrote a letter to their tech column asking about the Landau tops he had seen on a couple of NASCAR Winston Cup cars (this was back in the days when Stock Cars really started life as factory-floor "stock" cars) and if said Landau tops had conferred any sort of racing advantage.

They actually increase it! What they do is exploit the Bernoulli effect and create lift so the ball goes further away before coming down again.

While I don't question the conclusions of the researchers, I do doubt that comparing sharks to golf balls is a good analogy. I haven't read TFA so I don't know if the golf ball analogy is by the submitter or the scientists but at any rate, it seems wrong.

I was dubious about this science when I read the article, but I learned something in the end.From the article:

The team created artificial shark skin with a 16 x 24 array of synthetic scales, each 2 centimetres in length and angled at 90 to the surface of the "skin".They then placed the arrangement in a stream of water travelling at a steady 20 centimetres per second.

Shark scales are tiny - the crown is barely visible to the naked eye. So these scientists have scaled them up (so to speak) at least 2 orders of magnitude. With fluid dynamics the scale of a model can change everything, especially in the range of sizes they are working with here. I thought they should have substituted a more viscous fluid for the water in order to get a useful model. I thought maybe this was just preliminary work and they'd do a better study if their results suggested that it could be valuable."

But before flaming the Slashdot editors for trumpeting this study as a "discovery", I did a little Googling and quickly wound up at Wikepedia learning about Reynolds numbers. Turns out you can model turbulence pretty accurately as long as the Reynolds number stays the same. In this case the Reynolds number is proportional to both the size of the shark scales and the velocity of the water flow, so it can be preserved while the scales are made larger if the velocity is reduced proportionally.

Which is exactly what they did. They're studying sharks swimming at 80 km/hr.

80km/hr = 8,000,000 / 3600 cm/sec = 2200 cm/sec

Or, about 100 times faster than the flow rate they used in their model. Neat.

There's still a problem with this - The Kolmogorov scale [wikipedia.org] is all about the smallest scales at which turbulence can occur in a fluid. It is effectively a fundamental constant in a fluid (can fluctuate in time/space, but usually treated as a field constant). Now, according to this source [google.com.au], the K-scale in the ocean is in the order of 1 mm. This means that while vortices may form easily behind 2cm high "scales", they probably do not form so easily behind real shark scales which are an order of magnitude or two below 2cm in length. I believe this is what TFA meant in this part at the end:

Sergei Chernyshenko, an aeronautical engineer from Imperial College London, UK, describes the research as fascinating. However, he points out that while the team have shown the existence of vortices, they haven't yet quantified the extent of the effect on the shark's drag, which he thinks could be minimal.

Well, if you're talking about devices that cause turbulence for the sake of boundary-layer adhesion, vortex generators have been in use on aircraft for years. More recently, they have been adapted to automotive use. Take a look at the trailing edge of the Lancer Evolution IX's roof... It has a line of 8-9 (if you count the antenna) vortex generators.

In fact a sharkskin like surface was added to Stars and Stripes racing yacht. Stars and Stripes scored a 4-0 sweep in America Cup in 1987.The technology provided such a tremendous advantage that it was banned in subsequent years of America Cup

The technology provided such a tremendous advantage that it was banned in subsequent years of America Cup

That's something that's always chapped my hide. Instead of expecting other competitors in racing to improve, generally speaking we just ban the things that allowed them to win. This happens constantly in auto racing; SCCA moved the 240SX from E to D class where it had to compete with cars with vastly more power, because it was doing too much winning. They changed restrictions specifically to eliminate Ford's GT40 from LeMans because it won too much. Why not just set a maximum dollar value if you want to lev